1. Field of the Invention
The invention relates an exercise apparatus. More particularly, the invention relates to an exercise apparatus adapted for exercising the hamstring of an individual in an efficient and effective manner. The exercise apparatus is particularly adapted for facilitating strength training, injury prevention and/or rehabilitation for leg muscles.
2. Description of the Related Art
Running, or in particular sprinting, is a very common component of nearly every competitive and recreational sport. The ability to run, or sprint is a skill that is enhanced with training many systems, mental, cardiovascular, neuromuscular, and musculoskeletal. Competitive and recreational athletes with better ability to sprint or those that can sprint more often in their respective sport are rewarded in accolades and in professional sports financially. Therefore, this ability is important to train.
A method of training that has become popular amongst coaches and trainers is to functionally train individuals in a manner that is specific to their respective sport opposed to isolating muscles involved in that sport. This concept of training in a similar manner to how you will compete is intuitive but is easier said than done. Creative exercises have been developed and implemented into exercise regimens that mimic certain functional tasks demanded by the desired activity or sport such as pulling a runner to supra-maximal speeds during training for track. Fundamentally similar training methods which focus on specificity training have shown positive but limited results of improved performance in the execution of these functional tasks and the potential to reduce injuries particular to movements commonly practiced in a respective sport.
With running being of particular interest, it should be mentioned that hamstring injuries have been identified as some of the most common injuries to occur in sports requiring significant running and sprinting activities such as soccer, Australian rules football, American football and track. A study of English professional football (soccer) has shown that hamstring injuries account for 12-15% of all injuries sustained. The English premier football league reported a gross revenue of close to $3.8 billion in the 1999-2000 season with injuries alone costing as much as $144.7 million. Hamstring injuries are common occurrences in athletes and currently there is not a clear understanding of what factors predict this type of injury. Muscle strength, flexibility, fatigue, and neuromuscular control are some of the most common factors commonly thought to be associated with hamstring injuries. Additionally, these factors are critical components in enhancing sprint performance through sports specific exercises. Measures need to be taken in order to further understand hamstring injuries with the goal of reducing the amount of hamstring injuries occurring each year. The scientific literature is constantly evaluating exercise interventions that can be utilized both in the prevention and rehabilitation of injuries and performance enhancement.
Experts have determined the hamstring is best developed when worked in a similar motion to when it is mostly stressed. The motion in which the hamstring is stressed the most is during the running motion and unsurprisingly the majority of injuries to the hamstrings occur during the running motion.
Just having the hamstring lifting weight in a free weight motion is not optimal for training the hamstring. This motion does make your hamstrings stronger but what experts have agreed upon is that this type of strength training is secondary when performance, injury prevention and rehabilitation factors are considered. The hamstrings are rated, as well as other body parts, on how much power they are able to generate, not necessarily on how strong they are, i.e., not how much dead weight they can lift from a stationary position but how fast and often they can lift that weight. The relationship between strength and power is usually positively/directly related but the strongest people do not necessarily produce the most power. For example, Michael Jordan has one of the highest vertical jumps in basketball. He, however, might be weaker than the majority of NBA players according to how much weight he can lift with his legs.
The ideal method for developing power for a certain human motion is to work those muscles involved in the motion using the same motion you are trying to make stronger. Using a rough example, if you want to jump higher, add weights on your shoulders and start jumping. This same method holds true for your hamstrings and running. A way to accomplish this is to add resistance to the leg while the leg is in a running motion. While working the hamstring as weight is increased, the speed of the leg decreases. As it eventually becomes stronger, it will gain speed until the weighted leg moves just as fast as the original non-resisted leg. Once this is accomplished, it is then time to add weight. This process can be iterated indefinitely but the output will follow a steep production curve.
To avoid confusion some terms and phrases used throughout the present disclosure should be defined. It is recognized that there is currently debate when defining the differences among walking, running and sprinting. For the purposes of the present disclosure these terms have been defined based on the desired outcome of the individual while not ignoring mandatory mechanical characteristics seen at each respective speed category.                Stride: One gait cycle which begins when one foot strikes the ground and ends when the same foot strikes the ground again. (ipsilateral to ipsilateral foot strike)        Stance Phase: Phase of gait when the foot is in contact with the ground        Swing Phase: Phase of gait when the foot is not in contact with the ground.        Walking: Has two periods of double support in each gait cycle, meaning that both feet are in contact with the ground simultaneously.        Running: Has a period of double float (no foot is on the ground) with foot contact being near the rear or mid-foot. Energy is conserved during this movement.        Sprinting: Like running, also has a period of double float but the goal is to move the limbs as fast as possible with no regard to aerobic cost. Foot strike is at the forefront of the foot.        
Despite the high instances of hamstring injuries, the exact cause and timing is still unknown. There are two prevailing theories that exist as to the phase of gait in which hamstring strains occur. The first theory states that the late swing and early stance phases of sprinting are the most predominant phases of gait where hamstring injuries occur. During late swing, the knee is extending and the hip is flexed. The hamstring muscles are eccentrically contracting to decelerate hip and knee extension in preparation for heel strike. Lengthening the hamstring muscles during activation could induce an eccentric contraction injury. Directly following late swing, the hamstring muscles continue their activation and concentrically contract which, conversely, could induce a concentric muscle strain.
A case study presented recently by Heiderscheit et al. (2005) documented a hamstring injury while collecting kinematic data of an athlete running on a treadmill. The subject was running relatively fast at 5.36 meters/second and it was determined that his biceps femoris was strained just prior to foot contact in the late portion of the swing phase. The biceps femoris has been reported as being significantly injured more often than the other hamstring muscles at an incidence upwards to 80%. This case study supports the theory that hamstring injuries occur at this time in the gait cycle.
The second theory hypothesizes that injury is most likely to occur later in the stance phase at toe-off where the length of the hamstring muscles aren't at their longest but where the largest peak torque levels are observed. Like early stance, if injured during this phase, the injury would be concentric in nature due to the concentrically contracting hamstrings which are assisting in hip extension. Despite the evidence provided by Heiderscheit et al. (2005), the dismissal of this second theory would be premature. The first theory discussed may describe the majority of hamstring strains but current evidence cannot disprove the possibility of this second theory. Due to this reasonable second theory and a lack of evidence to disprove, this aspect of the running gait should still be considered an important aspect of preventing and rehabilitating hamstring injuries and properly training an individual. Late swing phase as well as late stance phase occur at significantly different phases in the gait cycle. Being unable to rule out either possible phase for hamstring injuries, it is mandatory to at least study these two distinct aspects of the gait cycle. With the gait cycle being cyclic in nature, all aspects of the gait cycle should still be investigated.
One single causative factor has not been identified as predominant when evaluating the injury mechanism of strained hamstrings. The current literature suggests that there are several contributing factors which can cause hamstring injuries. The primary factors are further discussed. Each factors contribution to a possible machine application is further commented on.
Low hamstring strength is theorized as being a cause of hamstring injury. A high ratio of quadriceps strength to hamstring strength has further been suggested to increase the probability of a hamstring injury. This, however should not necessarily suggest that a weakness in hamstring strength is needed to cause injury, but rather just a disproportional quadriceps to hamstring strength ratio. It has been suggested that eccentric muscle strength may be a more significant factor than concentric muscle strength as a determinant of injury due to the functional eccentric role of the hamstrings during running gait. Results from just testing eccentrically do not, however, confirm this theory. The same can be said about measuring muscle strength concentrically. With all these conflicting results it is very difficult to draw any strong conclusions on the proper way to strengthen the leg muscles.
When considering quadriceps and hamstring strengths as causes for hamstring injuries, it might be less appropriate and very limited to solely use concentric and eccentric strength evidence as predictors of hamstring injuries but more appropriate to look at previous strengthening program results when determining what factor muscle strength has on hamstring injuries. It is has been found that increasing eccentric strength can improve the ability of a muscle to withstand forces and subsequently not fail. Current training regimens may involve increasing lower limb strength in general and might induce excessive quadriceps strength which might lead to injury.
A program involving the antagonistic dynamic training of the quadriceps and hamstring muscles, simultaneously over the entire range of applicable forces and speeds these muscles might encounter might serve as the best method for training the hamstrings correctly to reduce injury rates. However, there is currently not a machine available on the market that is capable of fulfilling this recommendation.
Many neuromuscular events take place during the running gait in order to control hip and knee motion in late swing and provide hip extensor torque in early stance. Since running is a relatively fast motion, these events occur over a very short period. If control and coordination are inadequate, muscle strain injury might result. It has been suggested that a method for adequately training the hamstrings must include improving neuromuscular control of the leg during swing phase. If an error is made in the control of the swinging leg at times when high hamstring forces exist, a strain is possible.
During the swing phase, when hamstring muscles are eccentrically contracting and decelerating the lower leg, high forces are generated and if fatigue occurs an injury may result. Improper synchronization of the dual innervation pattern seen between the short head of the biceps femoris and the remainder of the hamstring muscles might introduce an injury mechanism. A mistiming on contraction of the biceps femoris due to fatigue might reduce the ability of the hamstring muscles to generate sufficient forces and lead to a hamstring injury Improving neuromuscular control of the leg during the swing phase is recommended for reducing the likelihood of incurring a hamstring injury. Actively assisting limbs along a predetermined trajectory has been found to increase neuromuscular control of the assisted limb when the limb is no longer in a controlled environment.
Individuals who exhibit poor neuromuscular control during injury prone movements have been trained to correct these neuromuscular deficiencies and consequently reduced their chance of injury. This has been documented thoroughly in the ACL (Anterior Cruciate Ligament) mechanism of injury for female athletes. Functional training was introduced to correct these neuromuscular deficits and injury rates diminished. These findings have not yet been applied to the hamstring muscle mechanisms of injury. This might be in part due to the lack of a proper machine to facilitate this functional training. Taking the above information into consideration, actively assisting the foot while mimicking the swing phase of running might help improve the neuromuscular control of the lower limb and further reduce the risk of injury to the lower extremity.
Muscle flexibility is said to reflect the muscle's ability to lengthen and absorb forces. It hasn't been established whether decreased muscle flexibility is a potential risk factor for injury or a consequence of other factors which lead to injuries. Conditioning the hamstring muscles by placing the leg in positions seen during running should able the leg to at a minimum absorb forces seen during those same positions when actually running. This would inherently decrease the risk of incurring a hamstring injury.
Properly training an individual to achieve their peak sprinting performance, while also reducing their risk of injury, demands a multifaceted approach involving all aspects of muscular training. Functionally exercising the lower limb as if it were sprinting might help improve lower limb strength in proper proportions. This might also help neuromuscular control of the limb while sprinting which may decrease injury rates and enhance sports specific performances. More research has been called upon to further develop knowledge on each of these respective factors and their relative contribution to hamstring strains. Recommendations have been made to incorporate all of these factors into preventative and rehabilitative strengthening programs. If exercise programs are implemented that positively affect these factors, the probability of an injury occurring or reoccurring might decrease.
With the foregoing in mind, a need for a training method and exercise apparatus to improve running performance while reducing the hamstring running injury mechanism has been established.